In this Phase I work, we propose to carry out the research necessary to develop, test, and characterize a sophisticated, by avalanche-photodiode (APD) arrangement as a compact, high-performance replacement for photomultiplier tubes used in flow cytomers. Development of a high- performance, monolithic, linear array (MLA) of avalanche photodiodes is the Phase I focus. Successful testing and characterization of this novel photodetector, as proposed here, will buttress the larger research plan to develop a sophisticated flow cytomer utilizing several exciting new technologies to dramatically increase flow cytometer performance and utility. It is anticipated that in advanced research the final envisioned devise will have multiple lasers, sophisticated focusing optics, MLA-APD photodetectors, and a compact, high performance readout system. This advanced development will result in a flow cytometer are not currently used with good efficacy. Exciting new flow-cytometry applications ranging from monitoring municipal water supplies for pathogens to agricultural applications for live stock health management will then be possible. PROPOSED COMMERCIAL APPLICATIONS: Commercially, flow cytometers have wide market acceptance for use a score diagnostic devices in pathology and immunology departments in many hospitals across the United States. Since the market is established for these devices, our high-performance, extremely compact device should find broad acceptance and provide immediate technological improvement in the diagnosis and science carried out with flow cytometers.